Adjustment of particle beam landing angle

Abstract

The invention relates to a method and apparatus for measuring The landing angle of a particle beam is adjusted by scanning the beam over two cylindrical beam target surfaces that are positioned close to the system axis. The output of a beam detector is differentiated and displayed on an oscilloscope. The relevant lens current is adjusted until one side of the scan indicates that the focal plane of the beam is at the center of the beam target. The angle of the beam is then adjusted with the relevant deflector until both sides of the scan are the same, indicating that the beam is accurately vertical.

We claim: 
     
       1. An apparatus for adjusting landing angle on a workpiece of a particle beam travelling along a beam direction substantially parallel to a system axis and substantially perpendicular to said workpiece, comprising: a detector disposed to intercept said beam;   electronic means for processing beam current intercepted by said detector; and   at least one pair of first and second beam target members disposed on a top surface of at least one support plate on opposite sides of said system axis, each of said beam target members having a flat support surface abutting said top surface, a cylindrical surface convex toward said system axis having a target length of said cylindrical surface, and having a radius of curvature lying in a radial plane extending along said target length and being substantially perpendicular to said system axis.   
     
     
       2. An apparatus according to claim 1, in which said at least one pair of beam target members comprises two pairs of beam target members disposed in first and second perpendicular planes containing said system axis. 
     
     
       3. An apparatus according to claim 2, in which said top surface is a top surface of a support plate having an aperture therethrough for the passage of said beam. 
     
     
       4. An apparatus according to claim 2, in which said top surface is formed from the top surfaces of first and second support plates, each having a plate top surface and disposed on said opposite sides of said system axis. 
     
     
       5. A method for adjusting a landing angle on a workpiece of a particle beam travelling along a beam direction substantially parallel to a system axis and substantially perpendicular to said workpiece, comprising: a) sweeping said beam in a first adjustment plane across first and second beam target members disposed on a top surface of at least one support plate on opposite sides of said system axis, each of said beam target members having a flat support surface abutting said top surface, a cylindrical surface convex toward said system axis having a target length of said cylindrical surface, and having a radius of curvature lying in a radial plane extending along said target length and being substantially perpendicular to said system axis;   b) detecting said beam in a detector disposed to intercept said beam;   c) processing a signal from said detector in electronic means for processing beam current intercepted by said detector to generate first and second differentiated beam signals; and   d) adjusting a focal plane of said beam until it intercepts one of said first and second beam target members, whereby a corresponding differentiated beam signal from said detector adopts a characteristic shape; and   e) adjusting said beam direction until both said first and second differentiated beam signals adopt said characteristic shape.   
     
     
       6. A method according to claim 5, further comprising the steps of sweeping said beam in a second adjustment plane perpendicular to said first adjustment plane across third and fourth beam target members disposed on a top surface of at least one support plate on opposite sides of said system axis, each of said beam target members having a flat support surface abutting said top surface, a cylindrical surface convex toward said system axis having a target length of said cylindrical surface, and having a radius of curvature lying in a radial plane extending along said target length and being substantially perpendicular to said system axis; and repeating said steps b) through e) of claim 5.   
     
     
       7. A method according to claim 5, in which said step of processing a signal comprises differentiating said signal from said detector and displaying said differentiated signal on an oscilloscope. 
     
     
       8. A method according to claim 5, in which said step of processing a signal comprises differentiating said signal from said detector, digitizing said differentiated signal to form a digitized signal, storing said digitized signal and calculating edge slopes of said stored digitized signal. 
     
     
       9. A method according to claim 6, in which said step of processing a signal comprises differentiating said signal from said detector and displaying said differentiated signal on an oscilloscope. 
     
     
       10. A method according to claim 6, in which said step of processing a signal comprises differentiating said signal from said detector, digitizing said differentiated signal to form a digitized signal, storing said digitized signal and calculating edge slopes of said stored digitized signal.